Sneaky Owls Use Stealth And Fly Silently
November 20, 2012

Researchers Study Owls To Learn About Stealth Technology

Alan McStravick for — Your Universe Online

Researchers are learning about stealth technology from a famous winged inhabitant in nature. Would you like to know just who-who they are taking their lead from? If you guessed the owl, well the “who” clue wasn´t exactly the most difficult.

The reason they are looking at the owl is for its ability to fly silently. It is able to do this because of the natural design of its plumage, which aids in noise reduction. This allows the owl to hunt in a relatively stealthy manner.

The study, performed by researchers at the University of Cambridge, England, focused on the wing structure of the owl in an attempt to better understand how it is able to dissipate any noise that might have been created through flapping during flight. The researchers believe they can apply their findings, ultimately, to the future design of conventional aircraft. They are presenting their findings this week at the American Physical Society´s (APS) Division of Fluid Dynamics meeting in San Diego, California.

"Many owl species have developed specialized plumage to effectively eliminate the aerodynamic noise from their wings, which allows them to hunt and capture their prey using their ears alone," said Justin Jaworski with the department of applied mathematics and theoretical physics at the University of Cambridge. "No one knows exactly how owls achieve this acoustic stealth, and the reasons for this feat are largely speculative based on comparisons of owl feathers and physiology to other not-so-quiet birds such as pigeons."

One of the challenges for the researchers was learning how this wing design was able to mitigate air turbulence as air flows over the wing. All wings, both natural and human designed, will create turbulent eddies as the air flows across it. Sound is produced when eddies cross the trailing edge of the wing. With a conventional aircraft´s wing, which is particularly hard across the trailing edge, the sound produced is significant.

The researchers recognized, in their study of the owl´s wing, three distinct physical attributes that they believe allow them to fly silently. The first is a comb of stiff feathers that are on the leading edge of their wing where the air begins to flow across. The second is a soft, downy material that covers the top of the wing. The third is a flexible fringe across the trailing edge where the air flows off of the wing. The researchers are trying to determine if their silent flight is produced as a result of one of these attributes or via a combination of all of them.

To try to make this determination, the researchers worked to develop a theoretical basis for the owl´s silent flight ability based upon the flexible fringe on the trailing edge of the wing. The trailing edge, in both natural and engineered wings, is typically the primary source for noise coming off of a wing. The team built upon previous owl research, which had suggested that the noise from the wing was not dependent upon air speed. This previous research showed there was a significant reduction of high frequency noise across a range that human ears are sensitive to.

The team employed mathematical models to demonstrate how elastic and porous properties of a trailing edge could be implemented on a wing so that aerodynamic noise would depend on the flight speed as if there were no trailing edge at all. “This implied that the dominant noise source for conventional wings could be eliminated,” said Nigel Peake, one of the researchers from the University of Cambridge. “The noise signature from the wing could then be dictated by otherwise minor noise mechanisms such as the roughness of the wing surface.”

The team presented their findings in a public talk at the meeting on November 18, 2012.